dof question

[j_buck]

i know this might be a stupid question, so sorry :-)

but wich rb lens has more shallow dof, the

250, 4.5

or

127 3.5 / 3.8

 

im using the pentax 67 too much lately and i wanted to buy something sharp and fast for my rb67.

 

thanks aot!

[david_henderson]

<p>You need to add a thing or two. Do you mean at the same viewing distance- taken from the same position ( so the fov of the 250 will be smaller). Or do you mean that you'd alter the viewing position so the two lenses would produce the same field of view? Or do you mean if you took shots from the same position and cropped the 127 image to replicate the field of view of the 250?<br>

Do you want to assume that all shots are made with the lenses wide open, or with the same aperture?</p>

<p>Bob Atkins has considerable expertise in this area. You might start by looking at his site www.bobatkins.com</p>

<p>You can use any decent online depth of field calculator to work out the answer to your question including variable answers to the issues I've indicated above.</p>

[chris_waller]

<p>To add to what David has said, all other things being equal, DOF varies inversely with the square of the focal length of the lens, i.e. the longer the focal length the shallower the DOF, assuming the same subject-camera distance and the same aperture.</p>

[j_buck]

thanks alot!

so for portraits, if i wanna get the shoulders out of focus, better is probably the 250 4.5.?

[craig_shearman1]

<p>"if i wanna get the shoulders out of focus, better is probably the 250 4.5.?"<br /><br />Yes.</p>

[jose_angel]

<p>David mentioned different scenarios, because there are different answers to your question.</p>

<p>If we assume that your interest is to get the <strong>very same framing</strong> of a head and shoulders shot, think that the larger aperture lens will provide you the shallower DoF. The focal length <em>per se</em> is not that relevant here.</p>

<p>So the 127 lens will give you a shallower DoF (as it is one full stop wider than the 250).</p>

<p>But don`t miss the "convenience" consideration; a 250 provide a higher magnification, but not as high as with a shorter focal length lens; the minimum focus distance is also larger. The 127mm lens can be focused at a much closer distance, so the final magnification is larger than the one on the 250... Think that you may need to use an extension tube on the 250 (making the RB a very long&cumbersome tool, BTW), while with the 127, you can probably took a straight shot.</p>

<p>At the end, it is merely matter of <em>perspective</em>. With either lens, the DoF is so shallow, so you really don`t need to work at full aperture. The 250 will give you a flatter, maybe more interesting look, the 127 could be a bit on the short side.<br /> Don`t miss that there is a "middle point" lens, the 150 (RB, 6x6) or 180 (RZ), that provide a good, reasonable relationship between magnification and perspective.</p>

[jose_angel]

<p>It may be a good idea to previously check the area covered/focus distance charts for different RB lenses, they also include the use of extension tubes (anywhere in the web, maybe at <a href="http://www.butkus.org/chinon/mamiya.htm">Butkus</a>). You`ll know then if e.g., the lens suits you for a tight head shot with or without tubes, and if so, which one could be needed.</p>

[j_buck]

thanks alot for the answers.

the thing is i was always amazed by the dof of the mamiya rz 2.8 lens and the dof i get from my pentax 2.8 lenses..

and since i love my rb, i wanted a similar effect.. so mby 127 od the way to go

ill look into everything:-)

[jose_angel]

<p>Do you have a f1.4 lens? (for film or digital full format, say Nikon, Canon, etc.).</p>

<p>If so, there is a trick that could work to help making you an idea. Just divide by two the aperture of the medium format lens, and you`ll find a similar look by setting it on the smaller format lens.<br /> This way, an f2.8 lens on your Pentax 67 will be equivalent to a f1.4 lens on a DSLR camera, a f/3.5 (f3.8) lens wide open will have a closer look to f1.8 on the DSLR. If we talk about f4.5 on 6x7, the equivalent on the smaller format could be f2.2 (aprox.).<br /><br /></p>

[chris_waller]

<p>My favourite for portraits is the 180mm lens. I used to have the 250 but found the focal length just a little too long.</p>

[rodeo_joe1]

<p>The easiest way to answer this question would be to use one of the many online DoF calculators and play with the numbers.</p>

<p>My Excel spreadsheet that calculates DoF, magnification, subject area and everything else I think I'll ever need to know, says the exact opposite to most opinions above except Jose's, as well as my own gut instinct. Off hand I would have said the longer lens, but computer says no!</p>

<p>The 127 lens at f/3.5 and 2.1 metres scale distance gives a DoF of 94mm. The 250mm lens at f/4.5 and 4.134 metres gives a DoF of 121mm. Those rather precise distances give the exact same subject area in frame. So the shorter lens wins at f/3.5, contrary to intuition.</p>

<p>DOF tables don't tell the whole story though. Also the OOF "bokeh" at equal distances in front of and behind the subject will be much more blurred with the shorter lens, because those distances are a greater fraction of the subject distance. This holds even if the lenses are stopped down to the same aperture number. Unfortunately the perspective rendering of the shorter lens won't be quite as flattering as a consequence of its closeness, but that's another issue.</p>

<p>At the same subject distance and grossly different magnifications, then the longer lens will win of course.</p>

[flavio_egoavil]

<p>Short answer: The 250 lens.<br>

Long answer:<br>

The bigger the "absolute aperture", the shallower the DOF can be. Aperture = focal length / f stop<br>

So for the 250 and the 127 lens:<br>

250 / 4.5 = 55.55mm<br>

127 / 3.8 = 33.42mm<br>

So the 250 has more potential for narrow DOF. But even then, let's consider a 100mm f2.8 lens and a 200mm f5.6 lens. Both have the same absolute aperture. However, the 200mm will give more perspective compresion; for an equal framing (equal angle of view), the background will appear bigger (against the foreground) than when using the 100mm lens. Thus the 200mm lens will give the appearance of shallower DOF.</p>

<p>Finally, the rule of thumb for 6x7 is that to compare with 35mm, you halve the FL and the aperture. So, for example the 90/3.8 lens is comparable to a 45/1.9 lens in 35mm. This also means another rule of thumb: stop down 2 stops more than in 35mm. Thus, f2.8 in 35mm would in theory be similar to f5.6 in 6x7 format.<br>

<strong>However</strong>, since the usable resolution on the 6x7 format is far higher than in 35mm, and also the expectations for resolution on 6x7 format are higher, and typically such negatives are enlarged more, in ACTUAL use one would want to stop down the lens even more. Or, in other terms, DOF on the 6x7 format is even narrower than what the above rule of thumb indicates. Thus, your typical 90/3.8 lens is more comparable to a theoretical 45/1.2 or even 45/1.0 lens on 35mm.</p>

<p> </p>

[jose_angel]

<p>Flavio,<br /> If we are talking about a given framing (say, head and shoulders), with two different focal length lenses, the focus distance should be obviously different. <br /> So don`t miss that distance increase DoF. Your rule applies if the focus distance and "framing" is the same.<br /> Even though the "absolute aperture" (I understand you mean the diameter of the entrance pupil) could be larger on the longer lens, DoF is increased by the longer focus distance.</p>

<p>About the background; if it appears proportionally bigger or not, I`d say it is another topic. We are talking about subject`s DoF, not background appearance. It could be interesting to keep the higher magnification on it, but some could prefer a more "identifiable" background, while keeping the same subject`s DoF.</p>

<p>And the "rule of thumb" is just this, a rough way to know an "equivalence". Aspect ratios are different, so it is somewhat difficult to know it with accuracy (and many times, almost useless). It is certainly easier to halve a number than to diminish a f-number by e.g., 2-1/3 of a stop.</p>

[rodeo_joe1]

<p>Flavio, I refer you to my previous post, where the maths (and practise) shows that <strong>the shorter lens will give a shallower DoF by some margin.</strong><br>

That's for the same area of subject framed by each lens.</p>

<p>Like I said, it's easy enough to check, simply by using one of the many online DoF calculators.<br>

http://www.dofmaster.com/dofjs.html <br>

The distances I gave previously will result in the same subject area being in frame for the two lenses in question, and are from the film plane-to-subject. Not from the front node of the lens.</p>